1. Field of the Invention
The present invention relates generally to manufacturing a sensor with an attached leadwire, and more particularly to a method for manufacturing a thin film strain gage with the attachment of the leadwire directly to the gage shim.
2. Description of the Related Art
One of the major limitations in the performance of strain gages is often the attachment of the leadwire to the gage. There is a variety of attachment techniques which have been used on strain gages which include soldering and brazing techniques, wire bonding, and welding. Each of these techniques has inherent limitations.
Soldering and brazing introduce a foreign metallic alloy into the joint between the gage and the leadwire. This may result in offsets with temperature and drift with time. Both soldering and brazing require fluxing to obtain good wetting with strain gage and leadwire materials. The fluxes may cause local damage to the gage or leads during the joining process. Removal of all flux residue is essential to avoid corrosion of the joint and drift with time. Both soldering and brazing produce a local mass of material which result in a stress concentration in both the leadwire and the gage. Thus, the most likely place for failure to occur is in the leadwire or gage material adjacent to the joint, and the joint may exhibit significant strain sensitivity.
Wire bonding can be used to overcome some of these problems. In general, no fluxes are required, and the quantity of foreign material added is small. This technique thus minimizes the thermal offset problems and drift with time. Because the mass of material in the joint is small, stress concentration is minimized in the gage and wires. In a wire bonded joint, the likely location for failure is in the joint itself. To obtain reliable joints, this technique requires a high degree of surface cleanliness and freedom from oxidation. In general, a precious metal such as gold is used to create the bond. With some gage alloys such as a platinum-tungsten alloy, the precious metal is frequently solid-soluble in the gage alloy resulting in drift with time and temperature and a change in the characteristics of the gage itself.
Welding of the lead wire attachment eliminates the addition of foreign material in the joint and generally results in a physically strong joint. These joints may be made using only heat such as fusion or autogenous welding, or using a combination of heat and pressure such as thermo-compression bonding or spot welding. Because of the temperatures involved in these processes, local microstructural changes occur in the gage and leadwire alloys both in and near the joint. Since welding is a fusion process, the joint contains new alloys made up of the components of the gage material and the leadwire material and material composition gradients. The joints also have relatively high residual stress levels. As a result, these joints are prone to corrosion damage and fatigue cracking. It is difficult to insure stability of such joints with time and temperature.
Accordingly, there is a need for a method of manufacturing a gage and leadwires with a high degree of precision and strength. Ideally, the attachment of the leadwire to the gage must be mechanically rugged, protected from strain, not load the sensing element when the lead wire is moved, be hermetically sealed, be stable with time, and be stable with temperature.